As a child, Rachel had an emotionally abusive father who constantly made her work out, demeaning her for her weight. Eventually, Rachel snapped and killed her father by setting the gym he owned on fire, burning her own back in the process. She was arrested and sentenced to 10 years in a psychiatric hospital.

In adulthood, Rachel began impersonating women to land personal training jobs at gyms, putting her clients on strict diets and exercise regimes. Whenever her clients failed to live up to her expectations, the evil Rachel would murder them so as to eliminate her "failures". Her latest client before the events of the movie was a man named Daniel, whom she kidnapped and duct taped to an exercise machine, forcing him to continue her regime before eventually killing him.

The movie's events saw Rachel taking on new client Annie Hogan (the film's protagonist) under the alias of Alexandra Hale. As per usual, Rachel put Annie on a strict diet and exercise schedule, and pried into her recent divorce from her husband Mark during their first workout together. The psychotic trainer soon became obsessed with training Annie, and began killing anyone she believed to be a threat to her goal. Her first victim was a waiter who had tried to convince Annie to get dessert while the two were at a restaurant, bludgeoning him to death with his motorcyle helmet. She went on to kill Annie's smug co-worker Curtis Smith when she learned from Annie that he had beaten her out in giving an important work presentation, having him look at her car after claiming that it wasn't working before repeatedly smashing the car hood over him. She later presumably hacked into his email account to send a letter of resignation to his boss.

The next day, Rachel called her at work just as she was about to make the presentation to wish her luck, giving Annie her first hint at her trainer's true nature. When Annie arrived late for a session with Rachel in favor of having lunch with her friend Tina Wallace, the evil Rachel punished her by forcing her to hold a heavy weight without any support, with Rachel almost dropping it on herself. Finally realizing how crazy Rachel was, Annie ran out of the gym, upsetting Rachel. She attempted to go to Annie's apartment to get her to join her in an early morning jogging session, but Annie refused. The next day, after leaving numerous messages for Annie, Rachel waited for Annie in her office until she arrived. When Annie tried to tell Rachel that she was dropping her as a trainer, the evil Rachel presented her with the contract she had signed at the beginning of their relationship and told her she never gave up on her trainees. Before leaving, however, Rachel gave Annie back the keys to her house that she lost, frightening Annie with the prospect that she had made her own copies.

As Rachel continued to stalk and harass Annie, she began investigating her and discovered that her claimed identity was false. After failing to get help from the gym owner Doug, Annie came home and nearly ate ice cream which Rachel had poisoned to kill her (but the attempt failed when her cat Gordon ate the ice cream first). Annie went to Mark for help, who informed her that there was nothing he could do to help her due to lack of evidence. After Annie left, it was revealed that Mark's new girlfriend was none other than Rachel, who killed Mark offscreen later. The next day at work, Annie was approached by Daniel's brother Richard, who had been searching for Rachel to find out what happened to Daniel. After calling Rachel and claiming to want to have a session to distract her, Annie broke into Rachel's house to investigate while Rachel chased Tina into the path of a speeding truck (though she ultimately survived her injuries). While in Rachel's house, Annie discovered folders she used to document all her clients, with Xs over all their faces to indicate she'd killed them. When Rachel suddenly arrived after learning of Annie's lie, she watched a videotape of her father making her exercise while insulting her as Annie hid in the closet, barely escaping without being caught. But later, Rachel realized that Annie had broken in when she found Daniel's X-ed out photo was missing from his folder.

In the film's climax, Annie prepared to go stay at Mark's place after having reported Rachel to a detective after learning her true identity. But before she could, Rachel killed the locksmith Annie had hired before going to Annie's apartment and knocking her out with a hammer (doing so after Annie had found Mark's body under her bed). Rachel then took Annie to an abandoned building and duct taped her to another exercise machine, telling her she would let her go if she reached their goal of her losing 10 pounds. When Annie tried to sympathetize with Rachel for her childhood abuse, the psychotic trainer put a knife to her throat and claimed that her father was a great man who helped people. Annie managed to escape by strangling Rachel with the strings of the exercise machine and using her knife to cut herself free, but as she tried to find a way out of the building, Rachel attacked her with a pole and began strangling her, delusionally saying that she cared about her. Annie fought back by hitting her with a barbell, causing Rachel to fall and impale herself on a spike. Annie tried to help Rachel when she began apologizing, but as she did, Rachel grabbed her in another attempt on her life. But Annie finally subdued her by kicking a nearby post, sending a glass sheet falling onto Rachel that appeared to kill her. But the last shot of the movie showed a bicycle stopping as Rachel went jogging with Richard, possibly implying that Rachel survived and was stalking Annie again.